The 500/450 is found mostly in Holland & Holland rifles. They changed to the 500/465 after the British Government banned 45 bores in India and the Sudan.
It is a very good calibre, the ballistic equal of the 450 3 1/4" and the 450 No2. It shoots .458 diameter bullets. If you can fine brass it is a great calibre.
I have killed 3 elephants and 2 buff with my 450 No2.
I have killed several deer, pigs and even a few coyotes with 350 grain bullets as well.